1. Field of the Invention
This invention relates generally to the field of magnetron sputtering and more specifically to planar magnetron sputtering apparatus having improved plasma-confining magnetic fields to increase sputtering efficiency with low aspect ratio targets.
2. Background of the Invention
Glow discharge sputtering is a well-known process and is widely used to deposit thin films of various kinds of ceramic and metallic materials onto the surfaces of objects or substrates. The glow discharge sputtering process is usually conducted in a vacuum chamber and in the presence of a sputtering gas maintained under relatively low pressure. The material to be sputtered and deposited onto the substrate is commonly referred to as the target and is mounted on a cathode inside the vacuum chamber where it is eroded or sputtered by a plasma or glow discharge that is established on the surface of the target material. The cathode and target are held at a strong negative potential, so that positive ions from the glow discharge bombard the target material and eject target atoms, which then condense on the surface of the substrate, forming a thin film or layer of the target material on the substrate. Other particles and radiation are also produced at the target and in the plasma that may also affect the properties of the deposited film.
While the sputtering yield, i.e., the number of target atoms sputtered per incident ion, depends on the energies of the incident ions, the overall sputtering rate depends on both the energies of the incident ions as well as the total number of ions bombarding the target surface during a given time period. Therefore, in order to maximize sputtering efficiency, it is desirable to produce and confine the ions and electrons in the glow discharge as close as possible to the surface of the target material. Towards this end, numerous kinds of magnetron sputtering apparatus, as they have come to be known, have been developed which utilize magnetic fields or "tunnels" to confine the glow discharge in close proximity to the surface of the target.
Most planar magnetron sputtering apparatus generally include plate-like or planar targets along with suitable magnet assemblies for producing plasma-confining magnetic fields adjacent the target. While numerous shapes and configurations of plasma-confining magnetic fields have been developed and used with varying degrees of success, it is common to shape a plasma-confining magnetic field to form a closed loop ring or "racetrack" over the surface of the target material. When viewed in cross section, the flux lines of the magnetic field loop or arch over the surface of the target, forming a magnetic "tunnel," which confines the glow discharge to the ring or racetrack shaped sputtering region. As is well known, an electric field created by a high voltage between an anode and the target/cathode acting in combination with the closed loop magnetic field causes electrons within the glow discharge to gain a net velocity along the racetrack, with the magnitude and direction of the electron velocity vector being given by the vector cross product of the electric field vector E and the magnetic field vector B (known as the E.times.B velocity). The shape of the predominate electron path defines the portion of the target material that will be sputtered.
Unfortunately, in most conventional magnetron sputtering apparatus having such ring shaped magnetic tunnels, the arched shape of the magnetic field over the target surface tends to force or "pinch" the electrons, thus the predominate electron path, toward the center of the tunnel. This pinching effect causes the plasma density and, therefore, the sputtering erosion, to be highest along the centerline of the tunnel. As the target is gradually eroded, the pinching forces tend to strengthen, causing stronger pinching, ultimately resulting in a V-shaped erosion groove in the target. The fraction of the target material that has been sputtered away by the time the bottom of the V-shaped erosion groove reaches the back surface of the target is referred to as the target utilization. In most prior art sputtering magnetrons, the target utilization is relatively low, in the range of about 20% to 30%. Since the commonly used target materials tend to be relatively expensive, such low target utilization is wasteful and increases the costs associated with the sputtering process. For example, although spent targets may be recycled and reworked into new targets, the time spent changing and reworking targets can be significant and in any event, increases the overall cost of the sputtering operation. Therefore, any significant increase in target utilization translates directly into cost savings, as the increased target utilization enables longer production runs and less downtime spent in reworking and replacing targets.
Over the years, inventors have attempted to increase the target utilization associated with such planar magnetrons by changing the shapes of the plasma-confining magnetic fields. For example, Morrison in U.S. Pat. No. 4,180,450, discloses a planar magnetron wherein the curvature of the convexly arched field lines of the magnetic tunnel are flattened in an attempt to reduce the plasma pinching effect described above. The patent issued to McLeod, U.S. Pat. No. 3,956,093, increases target utilization somewhat by enlarging the sputtering region over the target. McLeod accomplishes the enlargement of the sputtering region by using electromagnets to apply a variable magnetic bias field to the otherwise static magnetic field, thus oscillating the magnetic field over the center of the erosion groove. Other magnetron apparatus increase the erosion area by physically moving the entire magnet assembly or the target during the sputtering process.
While these foregoing prior art devices have improved target utilization to some degree, the improvements have usually come at the expense of decreased power efficiency, or have required the addition of separate or additional electromagnetic coils or the addition of complex apparatus to physically move the magnet structure or target. Besides increasing the overall cost of the sputtering device, the addition of large numbers of components into the sputtering chamber may poison the sputtered film with unwanted impurities if suitable precautions are not taken to insure that the additional components themselves do not sputter.
Welty, in U.S. Pat. No. 4,892,633, discovered another way to increase target utilization without having to resort to complex apparatus to oscillate either the magnetic field or the target, with all their associated disadvantages. Specifically, Welty teaches a method and apparatus for magnetron sputtering in which some of the flux lines forming the closed-loop magnetic tunnel change curvature from convex to slightly concave within the sputtering region. Welty's improved field shape reduces the pinching effect and allows more complete consumption of the target material. Unfortunately, however, the Welty system is not without its drawbacks. For example, the magnetic shunt used by Welty to pull down the magnetic field lines over the surface of the target, thus change their curvatures from convex to slightly concave also substantially weakens the strength of the magnetic field. This weakened magnetic field reduces the maximum allowable target thickness, which, of course, reduces the length of the production runs and increases the downtime spent replacing targets. Alternatively, stronger magnets could be used to compensate for the reduced field strength. Unfortunately, however, most sputtering magnetrons already use the strongest magnets available, so this option is effectively foreclosed.
Therefore, there remains a need for an improved planar magnetron sputtering apparatus that can achieve a substantial increase in target utilization without the need to add separate electromagnets or other complex apparatus to move the magnetic field over the surface of the target, and without the need to move the target itself. Ideally, such a planar magnetron should achieve an increase in target utilization by using a simple static magnetic tunnel field, but without weakening the strength of the magnetic tunnel and without unduly limiting the maximum allowable thickness of the target material. Prior to this invention, no such device existed.